1,2,3,6-Tetrahydrophthalic anhydride undergoes double bond isomerization to 3,4,5,6-tetrahydrophthalic anhydride at moderate temperatures. At higher temperatures, this anhydride disproportionates to produce cyclohexane dicarboxylic anhydride and phthalic anhydride. However, disproportionation of N-aryl-1,2,3,6-tetrahydrophthalimides has not heretofore been feasible, even though such disproportionation potentially is valuable for preparing a variety of herbicidal chemicals as well as for end products useful in other applications.